I know why Canon went with 18MP.

Recently, when Sony came out with the 24MP APS-C A77, everyone was looking at 100% crops proclaiming how bad the IQ was. Of course, when you look at a 100% crop on a 24MP camera, you are actually zoomed in more than you would be with a lower res camera, while there's actually nothing wrong with the IQ at print sizes.

Everyone (pixel peepers, I'm looking at you) compare photos at 100%. Going to 32MP would effectively result in reduced quality of 100% crops (because they are zoomed in more). Everyone would look at 100% crops from a 32MP camera and proclaim that the incredibly high pixel density has ruined Canon's IQ, while in actual fact they were just zoomed in more.

What Canon have done is chosen a resolution where 100% crops actually still look good.